A need exists for a substantially global radio telecommunication system that can provide communication services to substantially any point on or near the surface of the earth. For such a system to achieve widespread acceptance, it should be capable of operating with portable subscriber units. In order for subscriber units to have acceptable portability, they should be capable of low power battery operation, and they should be capable of transmitting and receiving electromagnetic signals through a relatively small antenna. In addition, such a system should use only portions of the electromagnetic spectrum which are allocated to it by governments within whose geopolitical jurisdictions the system is used.
In such a global radio telecommunication system, subscriber units may be placed in the control of system users, and the users may move their subscriber units to any place on or near the surface of the earth. In short, the system and those who operate the system may have no control over where the subscriber units are located. On the other hand, the system may be responsible for granting or denying particular communication services depending upon whether or not the system has received permission to operate at a point where a particular subscriber unit may be located. Moreover, the system may be responsible for billing in connection with the use of communication services, and the rates charged for such services may vary from location to location due to tariffs and the like.
A radio telecommunication system may carry out the job of granting and denying particular communication services and assigning particular billing rates to calls if it knows the locations of the subscriber units. Accordingly, it would be desirable to configure the system so that the locations of subscriber units may be determined and so that information describing locations may be transmitted to controllers which are responsible for making decisions regarding the granting or denying of communication services, billing rates, and the like.
Many prior art location determination systems are known, such as Global Positioning System (GPS), GLONASS, Loran, and the like. While subscriber units could be configured to incorporate components which take advantage of such location determination systems, these components would substantially increase costs of the subscriber units. Moreover, relying on such known location determination systems could reduce reliability of the radio telecommunication system by introducing reliance upon an external system.
The techniques used by such prior art systems to determine location could potentially be incorporated into the radio telecommunication system, but the introduction of such techniques could seriously degrade communication services. For example, many prior art location systems require the use of two or more transmitters or receivers ("locators") that are located at distant positions and that are capable of transmitting or receiving signals to or from a location to be determined.
The requirement for two or more locators to be within view over the entire globe, when combined with a global telecommunication capability, makes this approach unduly cumbersome. While this requirement might be met by placing satellites in high or geosynchronous orbits around the earth, higher orbits place satellites further away from subscriber equipment on the earth. This larger distance causes the subscriber equipment to consume excessive power or incorporate larger antennas just to participate in communication services. Moreover, higher orbits require increased spectrum allocation to carry a given amount of communications because the allocated spectrum may be reused less frequently in a given area.